Should I create a large map as a single model or in sections to export to Unity?
There are many ways to create large maps depending on the game project. You can finds lots of intricate information on YouTube searching for "GDC terrain" for example. The awesome AAA examples are often low-level implementations with many optimization techniques tailored to the specific games. Unity has their own optimized terrain system featuring texture painting, collider and foliage but not everyone opt to use it. 

Most terrain systems featuring an async continuous loading do it in square chunks. The game world is divided by the world coordinates and the terrain system keeps the nearest 4, 9, 16 or so terrain chunks around the camera position loaded. Choose your chunk size considering how far the player is able to see in general and in programming parametrize chunksize so that you can easily try different numbers to see which performs and works best for your game project.

Finally when it comes to Blender and modeling the terrain, consider what you want to store in the mesh data. You probably need a material with UV coordinate or you could use the world position XY in shader. You probably need multiple materials. You could do it with a splatmap texture or you could use vertex colors or even material slots if you don't need seamless transitions between materials. UV2 you should keep for lightmapping when working with Unity unless you have an alternative plan about lighting.

When transforming the terrain mesh remember that you don't want to disturb the chunk seams. Many games including WoW only transform the terrain in Z axis and any more complex geometry comes from additional meshes on top of the terrain. If you establish smooth rolling terrain using subdivision modifier, you can also get simplified collider for the terrain chunk by reducing subdivision level. This could be a clear starting point for a system to build up from.

Unity has vertex limit of around 64k per mesh so that might also limit your chunk size. I wouldn't split the mesh until exporting, you could set up a python script to do it for you where you could input your desired chunk size to tie in with what you want to use in Unity.

In the end this is a very open ended theme that depends heavily on your game idea and ties closely in with your game development environment.